Making LayoutUnit conversions to Float type explicit
LayoutUnit misuse, often from improper conversion to other types,is a 
constant source of bugs in Blink. This is the second patch with the goal 
of making such error-prone conversions explicit in the hopes of forcing 
developers to think through these conversions while writing their patches.
These conversion sites were discovered by toggling the float conversions
to explicit and finding the compiler errors.

BUG=350474

Review URL: https://codereview.chromium.org/200023002

git-svn-id: svn://svn.chromium.org/blink/trunk@169545 bbb929c8-8fbe-4397-9dbb-9b2b20218538